The field of the invention is utility apparatus, and the invention relates more particularly to manhole covers of the type used to permit access into electrical, telephone, sewer, and drainage lines and the like.
Manhole covers are invariably formed from cast iron because of its weight and strength. Such manhole covers must be equipped with some means for removing them from their frames, and this is most typically done by an opening through the manhole cover referred to as a pick hole. In order to remove the cover, an opening bar is inserted into the hole, and the upper end of the bar is pulled to provide sufficient leverage to lift the cover upwardly and permit it to be slid aside to permit entrance through the manhole.
For many installations, it is desirable to reduce the leakage of water, and other contaminants, through the manhole cover. For instance, for vaults used for containing electrical or telephonic equipment, water can be dangerous or harmful to the contents, and it is not uncommon for sufficient water to flow into a vault to make pumping the vault necessary before entrance can be possible. A major source of leakage through the manhole cover is the pick hole. Various attempts have been made to seal these openings, and hard plastic plugs have been used. Such plugs have proved unsatisfactory, however, particularly for manhole covers located in streets since the traffic tends to dislodge the plastic plugs from the hole Another disadvantage is ladies' high heels which can pass through the pick hole and provide a hazard. Another problem with sealing such holes is that since they are formed by a casting process, they are somewhat irregular in size and shape, and a close tolerance plug would not be satisfactory since hole sizes and shapes vary from cover to cover.